Question 19
(a)
Give one example of
a hadron:
a lepton:
[1]
(b)
Describe, in terms of the simple quark model,
(i)
a proton, [1]
(ii)
a neutron. [1]
(c)
Beta particles may be emitted during the decay of an unstable
nucleus of an atom. The emission of a beta particle is due to the decay of a
neutron.
(i)
Complete the following word equation for the particles produced in
this reaction.
neutron →
.................................... + .................................... +
.................................... [1]
(ii)
State the change in quark composition of the particles during this
reaction. [1]
[Total: 5]
Reference: Past Exam Paper – June 2016 Paper 21 Q7
Solution:
(a)
hadron:
neutron/proton
and
lepton: electron/(electron)
neutrino
(b)
(i)
proton: up up down or
uud
(ii)
neutron: up down down or
udd
(c)
(i)
neutron → proton + electron + (electron) antineutrino
{Note that
the decay of a neutron to produce proton is a known one and can be directly
inserted to obtain the answer.
Beta
particles are emitted. So, one of the blanks should be beta. Beta particles are
the same as electrons. So, we can write electrons instead of beta.
Beta
particles / electrons are examples of leptons. In an equation if a lepton is
emitted, its associated antimatter should also be emitted so that the overall
lepton number is zero.
For leptons,
a matter (e.g. electron, neutrino) has a lepton number of +1 while an
antimatter (e.g. anti-electron, antineutrino) has a lepton number of -1.
So along
with the electron, an antimatter should be emitted – either anti-electron or
antineutrino.
If an
antielectron is emitted, the overall charge on the left-hand side would be 0 (charge
of neutron) while that on the right-hand side would be (+1 – 1 + 1 =) +1 (due
to proton, electron and antielectron respectively). The charge is not
conserved. It cannot be an antielectron.
So, the
other particle is an anti-neutrino.}
(ii)
up down
down (quarks) change to up up down (quarks)
or
down (quark) changes to up (quark)
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